1. TECHNICAL FIELD
The present invention relates to a sheet type polarizable electrode body constituted by activated carbon fiber and binder and to method for making the polarizable electrode body, and further to an electric double layer capacitor using the polarizable electrode.
2. BACKGROUND ART
As a polarizable electrode material utilizing activated carbon, an instance wherein conductive agents such as graphite, carbon black or the like and a binder such as polytetrafluoroethylene are added to activated carbon powder to make a paste, which is then pressed onto a collector material such as punched metal foil, is well known. Besides, another instance wherein a conductive layer made by spraying of metal is provided on one side of an activated carbon fiber fabric has been proposed.
In that which uses the activated carbon powder as the polarizable electrode material, the bonding force between the metal collector and the polarizable electrode is weak and the activated carbon powder drops off from the collector or peels off therefrom or the bonding force between both members becomes weak during use due to stress caused by winding, and as a result, the internal resistance of the electric double layer capacitor gradually increases, thereby to deteriorate the collecting characteristic. Furthermore, when the polarizable electrode body consisting of the activated carbon powder and the binder is press-rolled onto the collector, its application efficiency is poor and there is a problem of divergence of capacity due to non-uniform application of the polarizabale electrode body. Moreover, when the activated carbon powder is used, forming of the electrode in a flat plate coin type structure is difficult. More concretely, during manufacturing of the polarizable electrode body, a press-forming can not be made unless the activated carbon powder has been preliminarily mixed up with electrolyte to form a paste. Therefore, in the case of an organic electrolyte wherein a very small water content will give adverse influence to the capacity characteristic is used, the polarizable electrode body must be sufficiently dried to remove the contained water.
Next, one wherein the activated carbon fiber fabric is used as the polarizable electrode can be easily made to a small type large capacity coin type capacitor. However, in the polarizable electrode body wherein a metal electrode layer 2 such as aluminum is formed on the activated carbon fiber texture 1 as shown in FIG. 1, conductivity in an a--a' direction in the drawing near the electrode layer is very good, but conductivity in the b--b' direction is poor. Accordingly, it is necessary that pressing with large force is made at during the sealing of the casing, so as to obtain sufficient physical contact among the fibers in the b--b' direction, to maintain the internal resistance in low state.
Furthermore, even when the activated carbon fiber texture of phenol type is used, when an specific surface area becomes above 2300 m.sup.2 /g (BET method), the strength is very much lowered and forming of the collector or the like with retaining the configuration becomes very difficult.